592 GENETICS AND EVOLUTION 



293. Heredity and Environment 



At one time a bitter argument raged as to whether heredity or en- 

 vironment is more important in determining human traits. It is now 

 clear that the two are interdependent and interact in many ways in the 

 development oi physical and mental traits. Some genes, tor example 

 the ones controlling the inheritance of the blood groups, produce their 

 effects regardless of the environment. The expression of other genes may 

 be greatly altered, even overcome or reversed, by environmental inffu- 

 ences. Our increasing knowledge of biochemical genetics suggests that the 

 greater the number of biochemical reactions there are interposed be- 

 tween a gene and its trait, the greater will be the opportunity for en- 

 vironmental influences to produce evident changes in the trait. 



It is sometimes stated, quite incorrectly, that if a trait has a genetic 

 basis, it cannot be affected by altering the environment; that is, inherited 

 diseases cannot be alleviated or cured by medical treatment. During 

 World War II experimenters reported that feeding large doses of vitamin 

 A would cure color-blindness. Vitamin A is a constituent of the light- 

 sensitive pigment of the cones, visual violet, and it was not unreasonable 

 that administering vitamin A might cure color-blindness. The gene for 

 color-blindness might in some way alter the cones so that a higher level 

 of vitamin A is required to achieve normal vision. The experiments 

 were repeated by other investigators, none of whom could demonstrate 

 any effect of vitamin A on color vision. The original authors had stated 

 that "since color-blindness is curable it is not the simple mendelian 

 trait popular theories assume it to be." The critics, who found negative 

 results, argued that the disease is inherited and therefore incurable. Both 

 of these arguments are incorrect, because inherited diseases can be al- 

 leviated. It now seems clear that vitamin A in the doses used will not 

 improve color vision, but the fact that color-blindness is inherited does 

 not preclude the possibility of finding some way to enable such people 

 to see color. If, for example, the color-blind gene blocks some step in the 

 synthesis of visual violet, supplying the substance normally made by this 

 step should "cure" color-blindness. 



Careful studies of monozygotic (identical) twins provide an estimate 

 of the relative importance of genetic and environmental factors in the 

 development of any particular trait. Identical twins, which develop from 

 a single fertilized egg, have identical genes; any differences between them 

 are due to environmental factors. Fraternal twins, which develop from 

 separate fertilized eggs, are no more alike than ordinary brothers and 

 sisters born separately. Identical twins are much more similar in intelli- 

 gence, as well as in a host of physical traits, than are fraternal twins; 

 indeed, identical twins reared apart are more similar in intelligence than 

 fraternal twins reared together. Children reared together in an orphan- 

 age, where the environment is fairly constant, show just as wide vari- 

 ability in intelligence as children reared separately in their own homes. 

 Even when children are adopted early in infancy, there is a much greater 

 correlation between the intelligence of the child and its true parents 

 than between the child and its foster parents. 



